Red Road (18)

For about three quarters of its length, Andrea Arnold's Red Road is as chilly and unsettling as current British cinema gets. Then in the final stretch, it starts obligingly dotting its 'i's and crossing its 't's and smoothing out its asperities, as its characters move from an extended state of unbearable tension towards reconciliation and (excuse the word) closure. Until Red Road becomes just another British film, it's an audaciously stark anomaly.

Perhaps Arnold, an Oscar winner with her 2003 short Wasp, suffered a failure of nerve; but what goes before is so hard-edged that I suspect a case of financier pressure. The fact is, films don't need to send viewers home relieved: the box-office success of Michael Haneke's Hidden shows that the more wobbly and perplexed we feel at the end of a film, the more likely we'll come back for repeat viewings.

Similarly, Lodge Kerrigan's recent Keane, a remarkable portrait of psychic meltdown, demonstrated that a film can provide the suggestion of last-minute redemption and still keep the viewers satisfyingly uncertain about whether they, and the characters, are off the hook. Not that I at all object to Red Road's closing use of Honeyroot's piano-ballad version of "Love Will Tear Us Apart", but it doesn't exactly leave us in any doubt that we have permission to leave the cinema smiling.

Ending apart, Red Road is a film with steel in its soul, and the most encouraging British debut in some time. For most of its length, you don't know where it's heading, or why, although Arnold provides enough discreet nudges for us to form a vague notion. It begins with a youngish woman scanning a field of TV screens, surveying everyday life on the streets of Glasgow. Although it's her work as a CCTV operative, Jackie (Kate Dickie) seems to be watching for her own delight, not as a detached voyeur but finding glee in the random strangeness of the populace. Every screen tells a story, and any one of Jackie's TVs could provide the narrative for this film: perhaps she'll make a connection with the man walking his weary bulldog, or the office cleaner shimmying to her earphones.

Instead, another chance sighting kickstarts Jackie's journey - a man screwing a woman against a wall. Jackie recognises him, but we don't know how: we only learn that he's named Clyde, and is recently out of prison. We see something of Jackie's oddly anaesthetised existence: she lives alone, has desultory sex with a lover in the front of his van, and has lost touch with an elderly couple who invite her to a wedding out of the blue.

Otherwise, all we know is that she's a woman who watches avidly, nostrils flaring as if inhaling the information.

Jackie begins to stalk Clyde (Tony Curran). She follows him to the bleak Red Road housing estate, tracking him so closely that she's practically treading unsensed on his heels: intent but in a kind of trance, she's a ghost drifting through a phantom city. She wanders into a party in his tower-block flat, where Clyde finally looks right at her, his hawkish face filling the screen. She's transfixed by him from the outset, even more so when he uses a chat-up line that I couldn't possibly quote in a Sunday paper (but don't be surprised if it catches on with a certain louche breed of male film buffs).

The revelation of what Jackie wants, and what she's prepared to do to get it, comes as a potent sting - at which point the film has perhaps achieved everything it has set out to. A potent female fantasy about a woman ambivalently taking power and letting go, Red Road is akin to (but far sharper than) In The Cut, Jane Campion's awkward upmarket variant on the erotic thriller genre: it's similarly about a woman who ventures out from a safe world into a darker realm of sexual menace.

Dickie and Curran make a magnetic duo - bony and taut, their Jackie and Clyde as suspicious and feral as each other. When Arnold finally brings them together, the encounter is intense and graphic, the realest sex you'll see in a British film - even though Arnold slightly dissipates the effect by over-stressing Clyde's Heathcliff aspect, making him a soulful hard man with a penchant for wood-carving and an ear for a fox's bark in the city night.

Once we know what Jackie's up to, all that remains is for Arnold to tell us why - which she does awkwardly by having one of the characters burst in demanding an explanation. After that, characters sorrowfully fill each other in on the back story, and find solace and redemption over a cup of tea and a scan through the family photos. Until then, Red Road has been too canny for such obviousness: in the main, it's a rare British film that communicates through action and atmosphere rather than through words. Its moods and thick visual textures are closer to the Dardennes or Michael Haneke (in the use of harsh-grained, nervously shuddering surveillance imagery) or even to Kieslowski in the harsh urban mode of A Short Film About Killing (to which a dangling car ornament is surely a grim nod). Robbie Ryan's DV photography paints a forbidding landscape in a spectrum of red, from murderous crimson to decrepit rust, the thick night crackling with ruby-like flares.

Red Road emerged out of a Scottish-Danish initiative called Advance Party, co-devised by sometime Dogme director Lone Scherfig, in which three film-makers were provided with a list of recurring characters. Arnold's opening episode of the trilogy is so powerfully self-enclosed that with all respect to the other two directors, you can't help feeling she's pretty much rendered the rest of the project redundant. I don't feel we need to know what Jackie or Clyde or the man with the dog does next: what Andrea Arnold does next is the really intriguing question.